A game of RF and gradient pulses: Obtaining an efficient homogeneous flip angle with strong inhomogeneous B1 fields

Abstract

Adiabatic RF pulses are useful pulses for inhomogeneous B1 fields caused by surface RF coils, however the increase in SAR will lengthen the TR, and possibly also the TE if the adiabatic pulses become too long. Using the superadiabaticity theorem the increase in SAR can already be reduced, making it possible to use an adiabatic TSE in humans at 7 T (chapter 2). However the length of the TR might still be too long to make the full adiabatic TSE clinically acceptable. On the other side, it is possible to design a 1D or 2D RF pulse which compensates the inhomogeneous B1 field and create a uniform flip angle distribution. Using a TOFU pulse in combination with a breast surface coil (chapter 3), the simplicity lies in the already present gradient and due to the more efficient spin energy distribution, the RF pulse will become less SAR intensive and making it compete with the conventional sinc pulses. The TOFU pulse does require a slab selective 3D sequence. Also, the direction of placing the slab selection has to align with the dominant inhomogeneous B1 field. If the B1 field of a coil is not dominantly inhomogeneous in one dimension, but in two dimensions, the TOFU pulse does not work anymore and other options should be considered. For compensating the radial inhomogeneous B1 field of a monopole antenna, a 2D RACE pulse is designed (chapter 4). The RF pulse works in the low flip angle regime and contains less SAR than adiabatic RF pulses. Therefore the 2D RACE pulse might be the best choice for the extreme inhomogeneous B1 field of the antenna. A comparison with conventional pulses have been shown in vivo in the human rectum. The duration of the 2D RACE pulse is determined by the hardware limitations of the gradients. The slew-rate and the maximal strength are limited in the sense that the RF pulse itself had to be stretched in order to suffice the hardware limitations, which makes the 2D RACE pulse relatively long. The endorectally inserted monopole antenna can boost the SNR substantially, not only for imaging the rectum, but also for MRI of the cervix (chapter 6). In this method the monopole is used as a receive-only coil. Combining the endorectal receiver with seven external dipole antennas on a multi-transmit platform, makes the setup capable of providing uniform excitations with high SNR. The local B1 shimming by setting the amplitude and phase of the external elements independently, is a mechanism to obtain a homogeneous B1 field in the cervix and its close surroundings. Using the monopole antenna to boost the SNR locally will give even more detailed information about the cervical structures. This setup makes it possible to image cervical cancer patients, which may, or may not, have parametrial invasion (cervical tumor growth in its surrounding connective tissue) in earlier stages. Detection of parametrial invasion may be used for further treatment planning (chapter 5)